1. Field of the Invention
The present invention relates to bulk acoustic wave resonators and, more particularly, to a temperature compensated bulk acoustic wave resonator with a high coupling coefficient.
2. Description of the Related Art
A bulk acoustic wave (BAW) resonator is an electromechanical device that sandwiches a piezoelectric structure between a lower metallic electrode and an upper metallic electrode. When an alternating electric field is placed across the piezoelectric structure by way of the electrodes, the piezoelectric structure mechanically deforms in a periodic manner and generates a standing acoustic wave.
One type of BAW resonator is a solidly mounted resonator (SMR). A SMR uses a Bragg acoustic reflector to reflect the acoustic wave. A Bragg acoustic reflector is constructed with alternating layers of low-acoustic and high-acoustic material, where each layer of material has a thickness that corresponds to one-quarter of the wavelength of the fundamental resonant frequency of the SMR.
One type of SMR uses dual Bragg acoustic reflectors to reflect the acoustic wave. An SMR with dual Bragg acoustic reflectors has a lower Bragg acoustic reflector that touches and lies below the lower metallic electrode, and an upper Bragg acoustic reflector that touches and lies above the upper metallic electrode.
In semiconductor applications, the piezoelectric structure is typically implemented with aluminum nitride (AlN), although zinc oxide (ZnO) or lead zirconium titanate (PZT) are also commonly used. The piezoelectric structure has a thickness that is substantially equal to one-half of the wavelength of the fundamental resonant frequency of the SMR.
For example, a SMR with dual Bragg acoustic reflectors that has a 2.5 GHz fundamental resonant frequency has a piezoelectric structure with a thickness equal to one-half of the wavelength of the 2.5 GHz fundamental resonant frequency. In addition, each layer in each Bragg acoustic reflector has a thickness equal to one-quarter of the wavelength of the 2.5 GHz fundamental resonant frequency.
One of the problems with constructing a higher frequency SMR is that as the fundamental resonant frequency of the SMR increases, the thickness of the piezoelectric structure decreases. The problem with decreasing the thickness of an AlN piezoelectric structure is that AlN does not begin to grow into a well-textured, highly-oriented crystal structure until the first 100 nm or so of material has been deposited.
As a result, an AlN piezoelectric structure which has a thickness in the range of 100 nm-200 nm would be expected to have a low K2 (electrical-acoustical coupling coefficient) as well as a low quality (Q) factor (a relatively wide range of frequencies centered on the fundamental resonant frequency), which degrades the needed performance of the device as a frequency reference.
Another problem with a SMR is that the SMR tends to have a relatively poor temperature coefficient of frequency (TCF), i.e., the fundamental resonant frequency changes a significant amount over temperature. One approach to reducing the TCF of a SMR to near zero ppm/° C. is to form a thin layer of oxide (e.g., 70 nm) that lies between the piezoelectric structure and the upper electrode. However, one problem with this approach is that the thin layer of oxide that lies between the piezoelectric structure and the upper electrode causes the K2 to fall from approximately 6.9% to approximately 4%.
Thus, there is a need for a higher frequency SMR that has a K2 above 6.5%, a high Q factor, and a TCF near zero ppm/° C.